1. Field of the Invention
A glass tube of an annular fluorescent lamp is manufactured in such a manner that a straight glass tube is heated/softened in advance, and this glass tube (to be referred to as a tube hereinafter) is wound around a forming groove formed in an outer surface of a bending drum (to be referred to as a drum herein after), thereby bending the tube into an annular shape.
2. Description of the Related Art
A technique disclosed in Japanese Patent Disclosure (Kokai) No. 60-177529 is known as a prior art of such techniques. This technique is characterized in that in a method wherein a lower end portion of a heated/softened straight tube is locked by a drum, and the drum is lifted while the drum winds up the tube to form the tube into an annular shape, a variable-speed motor for driving the drum is controlled by an operation controller, thereby setting the ascending speed of the drum at optimal speeds with respect to various types of tubes. According to such a method, bending of various types of tubes having different thicknesses, sizes, and qualities can be performed by simple resetting of speeds using a single apparatus. Therefore, manufacturing efficiency and yield can be increased.
However, in the above tubular glass tube bending apparatus, the two speeds of the drum, i.e., the rotational and ascending speeds are determined by one driving system (in this case, one motor). More specifically, the rotation of the motor serving as the driving system is transferred by a chain or the like so as to lift the drum, and the rotation simultaneously causes the drum to be rotated through a gear mechanism or the like. For this reason, although only one driving unit is required in this arrangement, the relationship (gear ratio) between the rotational and ascending speeds of the drum is fixed. Thus, throughout the process of winding glass tubes, the ratio between the rotational and ascending speeds of the drum is constant, that is, when the ascending speed of the drum increases, the rotational speed of the same also increases, and when the former decreases, the latter also decreases. As a result, a disadvantage arises in that some sorts of glass tubes are distorted at the start of the winding process and therefore cracked easily. Also, glass tubes cannot be bent with precision.
Further, when glass tubes having different sizes are to be bent by a single apparatus, bending drums must be changed because glass diameters or annular radii are different from each other. If the diameter of the drum is increased, the rotational speed with respect to the ascending speed of the tube is excessively increased, and hence processing cannot be smoothly performed.